This invention relates generally to synthetic fuels, and more particularly, to a process for producing and dedusting oil derived from oil shale, tar sands, and other solid carbon-containing materials.
In view of the recent instability of the price of and the open access to crude oil, researchers have renewed their efforts to find alternative sources of energy and hydrocarbons. Much of this research has focused on recovering hydrocarbons from solid hydrocarbon-containing materials such as oil shale, coal and tar sands, such as by pyrolysis or gasification to convert the solid hydrocarbon-containing material recovered therefrom into more readily useable gaseous and liquid hydrocarbons. Typically, synthetic oils, e.g., shale oil, tar sands bitumen, tar sands oil, heavy oils or the like, are produced by one of a variety of techniques including in situ, modified in situ and aboveground retorting processes or by hydrocarbon derivation from the inorganic/mineral components of these solid hydrocarbon-containing resources rock material such as through solvent extraction, for example. Consequently, these synthetic oils typically contain finely divided inorganic solids (called dust). The removal of such solids from the oil (called dedusting) greatly facilitates the transportation, refining and use of these oils.
Vast natural deposits of oil shale found in the United States and elsewhere contain appreciable quantities of organic matter known as "kerogen" which decomposes upon pyrolysis or distillation to yield oil, gases and residual carbon. It has been estimated that an equivalent of 7 trillion barrels of oil are contained in oil shale deposits in the United States with almost sixty percent located in the rich Green River oil shale deposits of Colorado, Utah and Wyoming. The majority of the remainder of the United States' oil shale deposits is contained in the leaner Devonian-Mississippian black shale deposits which underlie much of the eastern part of the United States.
Shale oil is not a naturally occurring product, but is formed, for example, by the pyrolysis of the kerogen in oil shale. Crude shale oil, sometimes referred to as "retort oil," is the liquid oil product recovered from the liberated effluent of an oil shale retort. The upgraded product of shale oil is often referred to as "syncrude."
Generally, oil shale is a fine-grained sedimentary rock stratified in horizontal layers with a variable richness of kerogen content. Kerogen only has limited solubility in ordinary solvents and therefore generally cannot be efficiently converted to oil by solvent extraction alone. In contrast, in pyrolysis upon heating the oil shale to a sufficiently high temperature, the kerogen or a major portion thereof thermally decomposes to liberate vapors, mist, and liquid droplets of shale oil and light hydrocarbon gases such as methane, ethane, ethene, propane and propene, as well as other products such as hydrogen, nitrogen, carbon dioxide, carbon monoxide, ammonia, steam and hydrogen sulfide. In such processing, however, a carbon residue typically remains on the retorted shale, thereby resulting in the loss of some of the valuable carbon content of the oil shale.
As a result of dwindling supplies of readily accessible and commercially useable petroleum and natural gas, extensive efforts have been directed to developing retorting processes which will economically produce shale oil on a commercial basis from these vast resources.
The process of pyrolyzing kerogen contained in oil shale to form liberated hydrocarbons (known as retorting) can be done aboveground in vessels known as surface retorts or underground in in situ retorts. In principle, the retorting of shale and other hydrocarbon-containing materials, such as coal and tar sands, comprises heating the solid hydrocarbon-containing material to an elevated temperature and recovering vapors and liberated effluent therefrom. However, as medium grade oil shale yields approximately 20 to 25 gallons of oil per ton of shale, for example, the expense associated with materials handling is critical to the economic feasibility of any such proposed commercial shale retorting operations.
In surface retorting, oil shale is mined from the ground, brought to the surface, crushed and placed in a vessel (a "retort") wherein the oil shale is contacted with a hot solid heat carrier material, such as hot spent shale, ceramic balls, metal balls, or sand, for example, or a gaseous heat carrier material, such as light hydrocarbon gases, to allow for the transfer of heat to the raw oil shale. The resulting high temperatures in the retort cause shale oil to be liberated from the oil shale leaving a retorted, inorganic material and a carbonaceous material, such as coke. The carbonaceous material can be burned by contact with oxygen at oxidation temperatures to recover heat and to form a spent oil shale which is relatively free of carbon. Spent oil shale which has been depleted in carbonaceous material can be discarded or, if desired, removed from the retort and recycled as heat carrier material. The liberated hydrocarbons and combustion gases can, in turn, be dedusted in gas-solid separation systems such as in cyclones, e.g., such as shown in U.S. Pat. Nos. 3,252,886; 3,784,462; and 4,101,412, electrostatic precipitators, filters, scrubbers, and granular bed filters, for example.
Generally, decrepitation of oil shale accompanies the retorting thereof as retorting results in the decomposition of a large portion of the kerogen content of the shale largely leaving behind the fine grain inorganics of the sedimentary shale. Consequently, during the retorting of oil shale, substantial quantities of minute shale particles become entrained in the shale oil so produced.
In addition, the use of hot spent shale as a heat carrier material can result in increased dust formation and thus aggravate the dust problem. Generally, rapid retorting is desirable to minimize thermal cracking of valuable condensable hydrocarbons. Shale dust is also emitted and carried away with the effluent product stream during modified in situ retorting as a flame front passes through a fixed bed of rubblized shale, as well as in fixed bed surface retorting, but dust emission is not as aggravated as in surface retorting.
Shale dust ranges in size from less than 1 micron to 1000 microns and is entrained and carried away with the effluent product stream. Generally, because shale dust has such small dimensions, much of it cannot be effectively removed to commercially acceptable levels with conventional dedusting equipment, such as cyclones, centrifuges or filters.
The retorting, carbonization or gasification of coal, peat and lignite and the retorting or extraction of tar sands, gilsonite, and oil-containing diatomaceous earth create similar dust problems.
After retorting, the effluent product stream of liberated hydrocarbons and entrained dust is withdrawn from the retort through overhead lines and subsequently conveyed to a separator, such as a single or multiple stage distillation column, quench tower, scrubbing cooler or condenser, where it can be separated into fractions of light gases, light oils, middle oils and heavy oils with the bottom, heavy oil fraction containing essentially all of the dust. Typically, as much as 65% by weight of the bottom, heavy oil fraction may consist of dust.
It is very desirable to upgrade the bottom, heavy oil into a more marketable light oil, but because the heavy oil fraction is laden with dust it is very viscous and cannot easily be piped through transport lines. Dust-laden heavy oil plugs up hydrotreaters and catalytic crackers, abrades valves, heat exchangers, outlet orifices, pumps and distillation towers, builds up insulative layers on heat exchange surfaces reducing their efficiency and fouls up other equipment. Furthermore, dusty heavy oil erodes turbine blades and creates emission problems. Moreover, dusty heavy oil generally cannot be refined with conventional equipment.
Efforts directed to resolving such dust problems have included the use of electrostatic precipitators and cyclones, located both inside and outside the retort. Electrostatic precipitators and cyclones, however, must be operated as high temperatures and the product stream must be maintained at approximately the temperature attained during the retorting process to prevent condensation and accumulation of dust on processing equipment. Maintaining the effluent stream at these high temperatures may favor detrimental side reactions, such as cracking, coking and polymerization of the effluent product stream, which tend to decrease the yield and quantity of condensable hydrocarbons.
Over the years various processes and equipment have been suggested to decrease the dust concentration in the heavy oil fraction and/or upgrade the heavy oil into more marketable light oils and medium oils. Such prior art dedusting processes and equipment have included the use of cyclones, electrostatic precipitators, pebble beds, scrubbers, filters, electric treaters, spiral tubes, ebullated bed catalytic hydrotreaters, desalters, autoclave settling zones, sedimentation, gravity settling, percolation, hydrocloning, magnetic separation, electrical precipitation, stripping and binding, as well as the use of diluents, solvents and chemical additives before centrifuging. Typifying those prior art processes and equipment and related processes and equipment are those found in U.S. Pat. Nos. 1,668,898; 1,687,763; 1,702,192; 1,707,759; 1,788,515; 2,235,639; 2,524,859; 2,717,865; 2,719,114; 2,723,951; 2,793,104; 2,879,224; 2,899,736; 2,904,499; 2,911,349; 2,952,620; 2,968,603; 2,982,701; 3,008,894; 3,034,979; 3,058,903; 3,252,886; 3,255,104; 3,468,789; 3,560,369; 3,684,699; 3,703,442; 3,784,462; 3,799,855; 3,808,120; 3,900,389; 3,901,791; 3,910,834; 3,929,625; 3,951,771; 3,974,073; 3,990,885; 4,028,222; 4,040,958; 4,049,540; 4,057,490; 4,069,133; 4,080,285; 4,088,567; 4,105,536; 4,151,067; 4,151,073; 4,158,622; 4,159,959; 4,162,965; 4,166,441; 4,182,672; 4,199,432; 4,220,522; 4,226,699; 4,246,093; 4,293,401; 4,324,651; 4,354,856; and 4,388,179 as well as in the articles by Rammler, R. W., The Retorting of Coal, Oil Shale and Tar Sand By Means of Circulated Fine-Grained Heat Carriers as a Preliminary Stage in the Production of Synthetic Crude Oil, Volume 65, Number 4, Quarterly of the Colorado School of Mines, pages 141-167 (October 1970) and Schmalfeld, I. P., The Use of The Lurge/Ruhrgas Process For The Distillation of Oil Shale, Volume 70, Number 3, Quarterly of the Colorado School of Mines, pages 129-145 (July 1975).
In addition, solvent dedusting processes wherein a dusty shale oil stream is mixed with a solvent or multicomponent solvent and solids settle out of the stream have found only limited success as the rate of solids settling and cost of solvent recovery have restricted the use of these processes.
U.S. Pat. Nos. 4,572,777 and 4,584,087, assigned to Standard Oil Company (Indiana), relate to methods for recovering bitumen from a carbonaceous feed and for recovering a carbonaceous liquid from a fines-containing carbonaceous liquid feed by extraction of the feed with a solvent having a predetermined solvency power. These methods involve the selection and use of a solvent having a predetermined solvency power so as to dissolve a predetermined portion of either (1) the bitumen content of the carbonaceous feed or (2) the carbonaceous liquid feed, and to leave undissolved a sufficient portion of the bitumen or other carbonaceous liquid to settle out from the solvent-dissolved bitumen phase or solvent-dissolved carbonaceous liquid phase. U.S. Pat. No. 4,699,709, assigned to Amoco Corporation, discloses a method for removing solid fines in an extraction of bitumen by extracting bitumen with a mixed solvent comprising at least one solvent with a solubility parameter higher than that of the bitumen and at least one solvent with a solubility parameter lower than that of the bitumen to obtain a bitumen laden solvent mixture which is treated to remove the lower solubility parameter solvent, precipitating undissolved asphaltenic bitumen and solid fines.
In these methods, the agglomerated fines settling rate can be adjusted by altering the solvent selected or, in a multi-component solvent system, manipulating the relative proportions of the solvent. Agglomerated fines settling rates of up to about 1 foot per hour are shown.
The use of non-polar solvents, such as alkanes containing from 3 to 9 carbon atoms, in such processes generally suffers as these solvents typically result in the process having relatively low fines settling rates and sludge compression rates. Further, the use of polar solvents, such as methanol, in such processes typically results in the requirement of substantially greater relative amounts of heat addition for recovery in reuse of the valuable polar solvents as the polar solvents typically have comparatively high heats of vaporization, e.g., methanol has a heat of vaporization of about 3 times that of hexane.
U.S. Pat. No. 4,180,456 discloses a process for recovering a premium oil from a slurry produced by high temperature hydrogenation of a solid, hydrocarbonaceous fuel, such as coal. Such premium oils contain a large amount of asphaltenes and 1000+.degree. F. material in contrast to retort solids which, for example, typically contain no more than about 1 to 3 weight percent asphaltenes.
In the process disclosed therein, a feed slurry resulting from high temperature hydrogenation of the solid feed is contacted with a nonpolar solvent, preferably a hydrocarbon and, more preferably, a C.sub.5 -C.sub.9 aliphatic or alicyclic hydrocarbon or naphthenic or paraffinic fraction of a coal liquefaction product containing less than 10 wt. % aromatics. Thus, no distinction is made between aliphatic and alicyclic hydrocarbon solvents. A solvent:slurry weight ratio between 0.5:1 and 5:1, with a preferred maximum weight ratio of about 1:1 and an especially preferred weight ratio of 0.8:1 are disclosed.
Further, the '456 patent points to the criticality of the Marangoni effect in the effective extraction of the feed slurry and in the selection of a solvent to obtain proper operation of the process (Example 6). Thus, dispersion formation is a critical factor in solvent selection for the process disclosed in the '456 patent as opposed to the selection of solvents and operating conditions to obtain desired settling rates to permit the effective and efficient operation of the dedusting operation.
These prior art processes and equipment have not been universally successful in economically decreasing dust concentrations in such heavy oil fractions to acceptable levels.